In this paper, we proposed a new digital watermarking method, called multi-channel Watermarking Technique (MCWT), for the printed images. It embeds a hidden pattern into the original exhibited one in the multiple color channels and prints the watermarked results on the real paper. Then, the designed optical lens can extract directly the embedded pattern without the need of any complicate computation. Some experiments were performed. The experimental results show that the embedded image should be hidden in both cyan and magenta channels with the 15-30% size of the dot area. Meanwhile, these dots in the different channels are shifted along their screening angles.